That was set up originally that way so that the entire metric system (SI) would be based on a single unit the Metre. That metre was supposed to be 1/10,000,000 of the prime meridian thru Paris. (Of course if he had measured that correctly Metres and Fathoms would be almost interchangeable.)

Since a metre is no longer defined by a platinum rod but by a certain number of wave lengths of a certain light frequency great accuracy is possible.

But please, knock yourself out:

“Beyond the simple wear that check standards can experience, the mass of even the carefully stored national prototypes can drift relative to the IPK for a variety of reasons, some known and some unknown. Since the IPK and its replicas are stored in air (albeit under two or more nested bell jars), they gain mass through adsorption of atmospheric contamination onto their surfaces. Accordingly, they are cleaned …

What is known specifically about the IPK is that it exhibits a short-term instability of about 30 µg over a period of about a month in its after-cleaned mass.[18] The precise reason for this short-term instability is not understood but is thought to entail surface effects: microscopic differences between the prototypes’ polished surfaces, possibly aggravated by hydrogen absorption due to catalysis of the volatile organic compounds that slowly deposit onto the prototypes as well as the hydrocarbon-based solvents used to clean them.[17][19]

Scientists are seeing far greater variability in the prototypes than previously believed. The increasing divergence in the masses of the world’s prototypes and the short-term instability in the IPK has prompted research …”